    Patent Reference No. 1: JP utility model application laid-open No. H06-697
Conventionally, compression bags are known in which an item for storage which is bulky as a result of containing air, such as clothing, bedding or the like, is contained, and then compressed by expelling the air contained therein, in such a manner that the item can be stored in a compact fashion. Further, bags (air cushion bags) are also known which are filled with air and can maintain an inflated state. The compression bag is provided with an opening for introducing and removing the item for storage, and this opening can be closed to seal off the interior of the bag hermetically, by means of a zip fastener or the like. Many compression bags are provided with deaeration valves having an air passage for expelling air from the interior of the bag. On the other hand, the air cushion bag is provided with a check valve having an air passage for injecting air into the interior of the bag.
There are check valves having a composition in which two valve films made of synthetic resin are superimposed on each other. In these valves, rectangular valve films are used, and an air passage is provided by seal sections formed by bonding the longitudinal side edges of the valve films together, and air is allowed to pass through the two valve films from an inlet to an outlet, and the air passage is closable by the two valve films that closely contact together.
This check valve has a simple structure, but there are drawbacks in slackening of the valve films and inverse flow of air. One of the reasons for this slackening is thought to be local instability in the tension of the valve films which occurs as a result of the fixed positional relation between the films in the aforementioned seal sections.
On the other hand, in one example of a device aimed at preventing inverse flow in a check valve having the aforementioned structure, the patent reference No. 1 proposes bending of the air passage. However, although this check valve makes it less liable for inverse flow to occur, it does not resolve the aforementioned problem, namely, the fact that the valve films are fixed together in the seal sections, thereby producing local instabilities in the tension of the valve films.
In view of the circumstances, it is an object of the present invention to provide a check valve, and a compression bag and an air cushion bag equipped with a check valve, whereby the tension of the valve film can be stabilized, and inverse flow of air can be prevented effectively in spite of a simple structure.